1. Field of the Invention
The present invention relates to a charge-coupled device and particularly to a charge-coupled device having an improved transfer efficiency at a low temperature.
2. Description of the Prior Art
Recently, infrared image sensors comprising a combination of an infrared ray detector of a quantum type and a silicon charge-coupled device have been developed. Such infrared image sensors are normally used at a low temperature approximate to a liquid nitrogen temperature (77K) so as to decrease dark current of the infrared ray detector and to obtain a satisfactory signal-to-noise ratio. Consequently, charge-coupled devices (CCD's) forming such image sensors need to be operated also at a low temperature. Conventionally, as a CCD used for such an image sensor, a buried-channel CCD (BCCD) having excellent characteristics such as high-speed operation, high transfer efficiency and low noise is utilized. Such a BCCD is disclosed for example in Bell Syst. Tech. J. Vol. 51 (1970) pp. 1635-1640 by R. H. Waldem et al.
FIG. 1(a) is a plane view of a conventional n channel BCCD of four-phase drive system and FIG. 1(b) is a sectional view taken along the line A--A in FIG. 1(a). First, the structure of the charge-coupled device shown in FIGS. 1(a) and 1(b) will be described. In FIG. 1(b), an n type impurity region 120 is formed on a p type silicon substrate 130, the impurity concentration of this region 120 being normally about ten times as high as the impurity concentration of the p type silicon substrate 130. On the n type impurity region 120, a gate oxide film 110 is formed and on the gate oxide film 110, a plurality of gate electrodes 21, 31, 41, 51, 22, 32, 42 and 52 are formed at predetermined intervals. In addition, as shown in FIG. 1(a), a transfer channel region 10 for transferring a signal charge is defined normally by providing a thick silicon oxide film or a high-concentration p type impurity region (not shown) in the peripheral portions of the substrate. To the gate electrodes 21 and 22, a clock signal of the first phase is applied through a clock bus line 70; to the gate electrodes 31 and 32, a clock signal of the second phase is applied through a clock bus line 80; to the gate electrodes 41 and 42, a clock signal of the third phase is applied through a clock bus line 90; and to the gate electrodes 51 and 52, a clock signal of the fourth phase is applied through a clock bus line 100.
In the following, the operation of the charge-coupled device shown in FIGS. 1(a) and 1(b) will be described. In the n channel BCCD having the above described structure, a potential to electrons in the n type impurity region 120 under the respective gate electrodes changes dependent on the gate potential. As the gate potential increases, the potential to electrons in the n type impurity region 120 decreases to reach a minimum value. The phases of the clock signals of the first, second, third and fourth phases applied through the clock bus lines 70, 80, 90 and 100, respectively, deviate from one another by .pi./4 so that the clock signals of more than one phase are always at a high potential. If the phases of the clock signals deviate in the order of the first clock.fwdarw.the second clock.fwdarw.the third clock.fwdarw.the fourth clock, a signal charge is transferred in the direction shown by an arrow in FIG. 1(a).
In such a BCCD as described above, with a low temperature approximate to a liquid nitrogen temperature enabling the infrared image sensor to operate, the transfer loss increases rapidly according to the fall of the temperature as shown in Japanese Journal of Applied Physics vol. 22 No. 6 (1983), pp. 975-980. FIG. 2 is a graph showing a relation between the temperature and the transfer loss in a conventional BCCD, in which the vertical axis represents the transfer loss and the horizontal axis represents the temperature. Such increase of the transfer loss is caused by capture of carriers to an impurity level in the n type impurity region 120, that is, a freezing phenomenon of carriers. Thus, in a conventional BCCD, the transfer efficiency is deteriorated at a low temperature due to such a freezing phenomenon, which constitutes a serious obstacle to the increase of picture elements of an image sensor.